Inorganic pigments such as iron oxides are commonly used in coloring plastics, such as low density polyethylene, by incorporation of the pigment with the resin by intensive mixing, such as by compounding in a Banbury or two-roll mill, or less intensive mixing such as twin-screw preplasticizing followed by extrusion, injection molding, or film blowing. A key performance property of the pigment is the dispersibility of the pigment in the resin, which is a measure of the ease with which the pigment can be intimately mixed with the resin. Large aggregates of undispersed pigment can cause surface specks, poor gloss, and color streaks on molded parts as well as being readily visible, and can cause weak spots and holes in blown film.
A pigment concentrate can be prepared by mixing a carrier resin, usually the same as or at least compatible with the eventual matrix resin, and the pigment at a loading of typically 40 to 50% in a Banbury or two-roll mill, followed by pelletizing. Later in use, the pigment is let down from the pigment loading to a concentration in the matrix resin of typically 5% or less, by mixing the concentrate with the matrix resin during melt processing, such as by extrusion or injection molding.
Additionally, in compounding and processing plastics, the material about to be extruded is normally passed through a screen to remove coarse particles. Large amounts of agglomerates will cause rapid screen blockage leading to low process throughputs and excessive equipment down time for screen replacement.
Several tests are used to determine pigment dispersion in a carrier resin. In the screen life test, compound is extruded through a screen, typically 150 or 325 mesh, and the time is measured until a predetermined pressure drop across the screen is achieved. This time represents screen life, with increasing time being desirable.
Another pigment dispersion test is the film quality test, which involves compounding pigment in resin, then letting down the concentrated compound in an extruder, and then blowing film from the extrudate. The film, typically 2 to 6 mils in thickness, can be visually examined for specks of undispersed pigment.
Also, X-radiography can be used to examine plaques of pigment concentrates to detect areas of insufficient pigment dispersion. On the radiograph, agglomerates of undispersed inorganic pigment, which have relatively high specific gravity, show as dark specks.
Surfactants generally have been used to enhance the processing of inorganic pigments in various respects. The following patents are typical.
U.S. Pat. No. 4,599,114 issued July 8, 1986, to Atkinson for "Treatment of Titanium Dioxide in Other Pigments to Improve Dispersability" discloses the use as a surfactant of the reaction product of a diamine, a carboxylic acid, and a fatty acid to improve dispersibility of titanium dioxide pigment in a resin medium.
U.S. Pat. No. 4,277,288 issued July 7, 1981, to Lawrence for "Fluidized Granulation of Pigments Using Organic Granulating Assistant" discloses the use of high boiling organic surfactants to promote agglomeration of pigment particles prior to dispersing the pigment in the final application medium.
U.S. Pat. No. 4,681,637 issued July 21, 1987 and assigned to Bayer AG, for "Process to Make Yellow-Brown Zinc Ferrite Pigments" discloses the use of trimethylol propane and methylhydrogen polysiloxane as grinding agents for zinc ferrites to improve color values and dispersibility.
U.S. Pat. No. 4,230,501 issued Oct. 28, 1980, to Howard et al for "Pigments Dispersible in Plastics" discloses pigments mixed with waxy materials to promote dispersibility of pigment concentrate in thermoset and thermoplastic resins.